Dez Chesterfield
Jay Hill laments the demise of multi-person video conferencing in the new Messages app (versus iChat).
This feature is still available in Messages. Choose Messages / Preferences, and go to Accounts. The iMessage account is only for iMessages and FaceTime; to use the multi-person video chat, you must also have an AIM account configured.

If you upgraded your existing computer to Mavericks, or migrated from a Mac with Lion or earlier to a new Mavericks Mac, your AIM account is probably still configured or migrated. But if not, just add one - click the + button, and choose AOL - and supply your iCloud email address and password - mac.com, me.com and iCloud.com all work, and you should use whatever address you previously used with iChat, so that all your buddies still work.
Having added/verified the AIM account, in the Messages app, choose Window / Buddies, and you will now see the familiar window you are used to, from the old iChat app, from which you can start multi-person video conferences as before.
This also applies to using other 'missing' iChat features, such as multi-person audio and screen sharing (which I often use, and find invaluable). The iChat Theatre feature, though, really does appear to be removed.
I am not sure why Apple has hidden these features. I really hope it doesn't portend the eventual removal of them altogether - maybe they, instead, will integrate these features better with Messages, and remove the reliance on AIM...

"Design is not just what it looks like and feels like. Design is how it works."
Steve Jobs

Thank you Stephen.... I COULD NOT figure this out for the life of me!!! I think that the Messages app is one the POOREST interfaces I have ever seen from Apple. It is obtuse, clumsy, and nearly impossible to figure out. Hidden features... Unanswered questions...

The real pathetic part about Messages is that it seems to have done away with the ability to show people photos and play them movies while you're in a video chat. It's called 'iChat Theatre' in the old versions and it worked like a charm. I think there is no other current product that does that so well and so gracefully. And they seem to have simply abandoned it. Utterly baffling to me.